1. Field of the Invention
The present invention relates to a system for measuring curvature of a patient's eye and, in particular, concerns a corneal topography system that includes a handheld unit for obtaining an image of a placido pattern reflected off of a patient's cornea.
2. Description of the Related Art
Over the past several decades, the increased use of surgical techniques and contact lens to correct vision problems has resulted in an increased need for data relating to the topography of the cornea of the eye. It is understood that, deformations in the cornea of a patient's eye are largely responsible for vision problems experienced by a particular patient. Specifically, the shape of a patient's cornea is a significant contributing factor to such common eye diseases such as myopia. Generally, an eye with perfect vision has a near spherical cornea so that incident light is defracted inward towards a focal point in within the eye. Variations in the shape of the cornea can result in light not being defracted into the focal point of the eye thereby producing visions problems for the patient. These eye problems are typically corrected by positioning a lens, either a spectacle lens or a contact lens, in front of the eye that is configured to be able to correct for the deformations in the patient's cornea which are causing the eye problem.
In the past, ophthalmologists determined the correction needed by a particular patient empirically by positioning a series of lenses in front of the patient's eye until their vision improved. However, as analytic techniques and instruments have become more sophisticated, mapping of the cornea to obtain the overall contour of the cornea, has become more common. Corneal topography data provides a treating physician with information as to the localized radius of curvature of a particular cornea. This allows the treating physician to more accurately select contact lenses and it also greatly aids a treating physician in correcting eye deformations through surgical techniques.
In the past decade, the use of surgical techniques to correct eye problems such as myopia, have become significantly more common. Techniques such as radial keratotomy and other well-known techniques, require that the treating physician have detailed information as to the configuration of the patient's cornea. With this information, the treating physician can then cut, ablate, or otherwise change the outer surface of the cornea at various locations to alter the overall shape of the cornea to thereby correct the patient's vision. In fact, these techniques have become significantly advanced so that treating physicians are able to correct significant nearsightedness or far-sightedness to near perfect vision.
It is, of course, understood that the treating physician will need detailed corneal topography information to perform these surgical techniques and also to fit contact lenses in specific situations. As a consequence, corneal topography systems have been developed which provide detailed information about the topography of the outer surface of a patient's cornea. One such system is disclosed in U.S. Pat. No. 5,863,260 to Gersten et al. The system disclosed in U.S. Pat. No. 4,863,260 is typical of most currently available corneal topography systems. Specifically, corneal topography systems generally project into the patient's eye, a placido image which is an image of a plurality of concentric rings or mires. The image of these mires is reflected off of the patient's cornea and is then captured using a camera. Hence, the camera obtains a two-dimensional image of the mires being reflected off of the patient's three dimensional cornea. The position of the reflected mires in the captured image can then be used to calculate the curvature of the patient's eye.
Specifically, it is assumed that a cornea having perfect vision will be generally uniformly spherical. If the placido image was reflected off of a perfectly spherical surface, the reflected mires would appear on a two-dimensional image as a plurality of concentric rings with the two dimensional locations of the rings being related to the curvature of the spherical surface. If, however, the patient's cornea is not perfectly spherical, the positions of the plurality of mires in the resulting reflected image are generally displaced from the corresponding position of the mires that is reflected off of the perfect sphere. A comparison of the position between the image reflected off of the patient's cornea and a corresponding perfect sphere, will permit the determination of the deviation of the patient's cornea from a perfect sphere. In this manner, the radius of curvature of the patient's cornea at locations over the entire surface area of the patient's eye can be calculated thereby providing the topography of the patient's cornea.
It is understood that to accurately calculate the corneal topography data of a patient's eye, it is desirable that the patient's eye be positioned in a specific orientation with respect to the placido projector and the camera that is obtaining the image. This specific placement is required as the corneal topography data is obtained by comparing the image of the patient's cornea to an image that is reflected off of a corresponding calibration sphere. For the comparative analysis to be accurate, it is desirable that the patient's cornea be located in the same or corresponding orientation as the calibration sphere was when the calibration data was obtained. Consequently, most corneal topography systems include mechanisms for ensuring that the cornea is in the correct orientation with respect to the placido projector and the camera. In most systems, the placido projector is positioned so that the axis of the placido projector intersects the apex of the patient's cornea, i.e., is coincident with the optical axis of the patient's cornea. Similarly, the camera that receives the reflected image is also located so as to receive the image along the optical axis which intersects the apex of the patient's cornea. Moreover, the placido projector and the camera are also positioned along the optical axis so that the apex of the patient's eye is positioned a known distance from the camera. This known distance corresponds to the distance that the camera was positioned from the apex of the perfect sphere when the corresponding calibration data was obtained.
It will be appreciated that the requirements of specifically positioning the patient's cornea with respect to the corneal topography machine has resulted in the corneal topography machine being a very large and complex instrument. Specifically, complicated mechanisms are usually attached to the placido projector and camera so that an operator can accurately locate the placido projector and camera in the correct orientation with respect to the apex of the patient's cornea.
A further difficulty with prior art corneal topography systems is that the placido projector generally has to incorporate numerous lamps so that the placido projector is uniformly illuminated over its entire surface area. In some devices, the placido projector is comprised of a generally conical or parabolic projector that is six or eight inches in diameter at its outer end. Inside of the cavity defined by the projector, a plurality of concentric opaque rings and mires are formed on a translucent background. The light sources must be located so as to uniformly illuminate the translucent material so that the placido image can be projected onto the patient's eye. It will be appreciated that the light system used to illuminate such a placido projector requires a significant amount of space and also adds to the complexity of the corneal topography system.
For these reasons, corneal topography systems are generally expensive and complex systems that occupy a significant amount of space in an ophthalmologist's office. The room occupied by the corneal topography system and the expense of the systems generally limits the number of systems that are purchased by an ophthalmologist. Consequently, patient treatment is often bottlenecked by the existence of a limited number of corneal topography systems.
Moreover, it would be desirable to be able to asses corneal topography during or immediately following eye surgeries. As discussed above, nearsightedness and farsightedness are often corrected by changing the shape of the cornea. This typically requires that the patient be prone on an operating table during the surgery. In this position, it is generally not possible to obtain any corneal topography data. Further, the patient is generally not in a condition where they can be taken to another location to obtain the corneal topography data during the actual surgery. Hence, while existing corneal topography systems have some utility for surgeons engaged in vision correction procedures, the utility is limited by the lack of portability of the systems.
From the foregoing, it can be appreciated that there is a need for a corneal topography system which is smaller and more portable. To this end, there is a need for a corneal topography system that has a handheld component which can be carried by an operator while still allowing the operator to precisely located the handheld unit in the desired orientation with respect to the patient's eye. To this end, the handheld unit should include a placido projector with a light source that is sufficiently lightweight and compact so as to allow. The operator to correctly position the placido projector in the desired orientation with respect to the patient's eye and maintain the projector in this orientation while capturing the reflected image. The light source should, of course, still uniformly illuminate the placido projector so as to produce a high quality image on the patient's eye.